One of the blending components to be used in a refinery gasoline pool is cracked naphtha. Cracked naphtha contains both sulfur and olefins. The sulfur, which may be present in amounts that are about 0.3 wt.% or larger, is both a potential air pollutant and a poison or toxic material to the catalysts that might be used in the catalytic muffler of an automobile engine's exhaust system. On the other hand, the olefins, which may be present in an amount of about 30 wt.% or larger, have octane numbers that are higher than those of the corresponding saturates.
Today, sulfur dioxide that is generated by the burning of high sulfur fuels has been identified as one of the chief air pollutants. Hydrodesulfurization is an important method for producing fuels with relatively low sulfur concentrations and commercial hydrodesulfurization plants for treating fuel oils are now in operation to provide fuel oils that have legally acceptable sulfur levels. At this time, maximum sulfur contents of motor fuels have not been established by the government; however, the situation is changing rapidly. Restrictions on sulfur contents of motor fuels seem inevitable. The sulfur concentrations of blending components for the refinery gasoline pool and, hence, of cracked naphtha will have to be reduced.
Therefore, if the cracked naphtha is to be desulfurized without eliminating or seriously reducing the amount of olefins that are present therein, the desulfurization process that is used must be very selective, i.e., capable of removing substantially all of the sulfur without severely saturating the olefins that are present. Currently, there are several desulfurization catalysts that find considerable use in the petroleum refining industry. Such desulfurization catalysts include cobalt and molybdenum and their compounds on a suitable support, nickel and tungsten and compounds thereof on a suitable support, and nickel and molybdenum and compounds thereof on a suitable support. The support, in general, is the non-acidic or weakly-acidic catalytically active alumina. Such conventional desulfurization catalysts are selective, that is, these catalysts not only remove sulfur from the petroleum hydrocarbon stream being treated, but also tend to restrict the saturation of olefins in that petroleum hydrocarbon stream.
It must be pointed out that while such conventional desulfurization catalysts have good selectivity, there now has been found a catalyst that provides unexpectedly better selectivity.
Cole, in U.S. Pat. No. 2,392,579, discloses a process for treating olefinic and sulfur-bearing gasolines to effect substantial desulfurization and refining. A portion of partially treated product is recycled to maintain a small concentration of olefins in the hydrogenation reaction zone to prevent to a certain extent the undesired hydrogenation of normal olefins and aromatics. Cole teaches that the catalyst employed may be any of the known conventional super-active hydrogenation catalysts and composites thereof which may or may not contain such materials as alumina, magnesia, silica, zinc oxide, chromium oxide, etc., as stabilizers, promoters, or supports. Cole requires olefin recycle and does not provide any specific examples of the catalyst of the present invention.
Haensel, in U.S. Pat. No. 2,770,578, discloses a process for treating unsaturated and sulfur-containing stocks to obtain saturated and substantially sulfur-free charge stocks for other processes, which process employs two distinct catalysts, a hydrogenation catalyst comprising platinum and/or palladium, preferably combined with a carrier of silica, alumina, zirconia, titania, activated carbon, magnesia, or combinations thereof, and a sulfur-resistant desulfurization catalyst, such as a Group VI metal and an iron group metal on a suitable support, such as those employed in the hydrogenation catalyst. Haensel teaches that the process first uses the hydrogenation catalyst to saturate the unsaturated compounds in the feedstock at a temperature which is too low to effect desulfurization and then desulfurizes the saturated, unpolymerizable stock that is produced. Haensel wants saturation of unsaturates and removal of sulfur and does not give examples of the catalyst of the present invention.
Lefrancois, in U.S. Pat. No. 3,269,938, teaches a hydrogenation process employing a catalyst comprising molybdenum and nickel on a particular support of silica-magnesia to produce a product having a lower degree of unsaturation. The Lefrancois patent teaches that the process is particularly suited for the hydrogenation of low-quality kerosene. The Lefrancois patent does not disclose a process for the desulfurization of a cracked naphtha without substantial saturation of the olefins contained in the cracked naphtha. The patent does say that the process may be used to selectively hydrogenate any diolefins present in a catalytically cracked gasoline to monoolefins.
Gislon, et al., in U.S. Pat. No. 2,853,429, disclose a desulfurization catalyst that contains a Group VI metal, a Group VIII metal, and magnesia. It does not teach, disclose, or suggest the selective desulfurization of cracked naphthas or, for that matter, the selective desulfurization of any feedstock. In Examples 4 and 6, a straight-run gas oil is employed. In Examples 5 and 7, a catalytic cycle stock, having a boiling range of 215.degree. C. (419.degree. F.), to 320.degree. C. (608.degree. F.) and a sulfur content of 1.9% sulfur, is used. Neither of these feeds are cracked naphthas, as described hereinafter. Moreover, there is no indication of the presence of olefins in either the feedstocks or the products of these examples.
Eng, et al., in U.S. Pat. No. 3,475,327, disclose a process for the hydrodesulfurization of blended feedstocks. The blended feedstocks may contain virgin or straight run naphthas, coker naphthas, steam cracked naphthas or pyrolysis gasoline, catalytic gas oils, coker gas oils, and straight run gas oils. The process of Eng, et al., comprises contacting the feedstock with a catalyst comprising a mixture of a member of the group consisting of Group VI oxides and sulfides with a member of the group consisting of iron, cobalt, and nickel oxides and sulfides deposited upon a porous carrier, such as alumina, silica-alumina, bauxite, kieselguhr, magnesia, or zirconia. Eng, et al., indicate that a preferred catalyst is cobalt molybdate on a silica-stabilized alumina. This patent limits the amount of cracked naphtha that may be present in the feed being treated by the disclosed process. It does not disclose the treatment of a feed that is one hundred percent cracked naphtha. While this patent mentions a large number of porous carriers that may be used in the catalyst, it does not provide any specific examples of the catalyst of the present invention.
Conway, in U.S. Pat. No. 3,956,105, discloses processes for the hydrotreating of various hydrocarbons and mixtures of hydrocarbons, the catalyst that is employed in such hydrotreating processes, and the method for preparing such catalyst. Conway teaches that the catalyst constitutes a Group VIB component and a Group VIII component and a porous carrier material, and may contain a halogen and/or an alkali or an alkaline earth metal. Various porous carriers are considered as the carrier for the catalyst. Conway suggests that the carrier material is a refractory inorganic oxide, either alumina in and of itself or in combination with one or more refractory inorganic oxides, and particularly in combination with silica. While magnesia is mentioned as one of the porous carrier materials available or suitable, Conway does not specifically provide in any example an exact catalytic composition of the catalyst employed in the present application.
Meyer, in U.S. Pat. No. 3,764,519, discloses processes for the hydrocracking and hydrodenitrification of hydrocarbon fractions. They employ a catalyst that comprises an alumina-silica-magnesia matrix containing a hydrogenation component and a crystalline zeolitic molecular sieve substantially free of hydrogenation components and dispersed in the alumina-silica-magnesia matrix.
The catalytic composition that is employed as the catalyst in the process of the present invention appears to be an exceptionally selective catalyst for the desulfurization of cracked naphthas.